There WILL be some performance hit... the question is to what degree and if it will be noticeable. The question you should be asking yourself is if the benefits of using Parallels/VMware Fusion/VirtualBox worth the hit.

There are plenty of guides and tips here on the forums and on the internet on how to configure your VM for optimal performance. It really comes down to your hardware, settings, and what you intend to do in your virtual OS.

In my experience, you can get high performance from both your Mac OS and virtualized Windows if you use the right settings AND have plenty of RAM. I have a 2010 MBP with 8GB RAM and don't see any significant performance hit on Mac and an acceptable sluggishness on Windows.